This invention relates to systems for handling precision elements for inspection, and more particularly to systems for transporting semiconductor wafers between supply, inspection and collection points with extremely precise positioning and automatic control.
In the chip manufacturing phase of the semiconductor industry, the chips are defined on a large silicon or other crystal wafer in the form of a disk now often six inches in diameter and circular outline except for major and minor flats on the periphery. This wafer receives a rectangular array of small identical patterns, each including the minute lines, active elements and electrical components that make up a specialized or general purpose chip in the present state of the art. Many hundreds of patterns can be disposed on a wafer and each must be laid down with submicron precision, inasmuch as line widths and elemental areas are now very small. It is evident that when successive layers are to be built up for each pattern on the wafer, they must be carefully checked, at least on a statistical basis, before further processing can be undertaken. For example, when a pattern is to be laid down, a layer of photoresist is first disposed on the wafer, and then each pattern area receives an exposure, as by a step and repeat optical system. After the exposure and subsequent fixation of the photoresist material, unfixed material is washed off for the next deposition step. At any or all such steps optical inspection may be used to determine if defects have been introduced from misalignment, faulty chemicals, impurities or other causes. In many instances corrective steps may be taken, while in others only parts of a wafer may be used. In the extreme cases, the wafer is discarded as not suitable for further processing. Despite the automation of many other procedures within the semiconductor industry, optical inspection by trained operators or with highly precise equipment remains an integral and essential procedure.
Optical inspection of semiconductor wafers demands precision placement of the wafers relative to the optical axis of a high resolution microscope. The wafer has precise edge flats defining positional references for the patterns on the wafer, so that particular patterns can be located from these indicia. Initially, the industry used manual placement of the wafer on a precision stage manually movable in two directions under an optical microscope mounted on an cantilever support and viewing along an inspection axis. The inspector would then use fine positioning controls to shift the stage in X and Y directions to bring the desired pattern into view. The problems with this were many, starting with the danger of dropping the wafer. In addition, manual positioning was time consuming, not as accurate as desired, and the mechanisms that gripped the wafer would mar a portion of the surface, introduce contamination, or both. As the resolution of images steadily increased, any contaminating particulate that settled on a pattern might be of large enough size to introduce an invalidating defect in the pattern or even the entire wafer. Smaller pattern sizes and line widths materially increase the contamination danger, because the most carefully designed and built belt and pulley mechanisms can give off minute but interfering matter.
Since the initial efforts there have been devised a number of semiautomatic wafer handling techniques. Wafers are now stored and transported in cassettes, which have storage slots, open at one side, in each of which slots a wafer is disposed, spaced apart from adjacent wafers. A transport mechanism can then probe into a given position on the cassette, lift a wafer from its storage location and transport it to the inspection stage, at which the inspector can then finely position and inspect without handling the wafer itself. This alone, however, is time consuming because of the many thousands of discrete positions within each chip pattern.
Some inspection procedures now are substantially automatic, as for example in the measurement of linewidth. If a given line in a particular pattern on a wafer is presented at the optical axis, a specialized linewidth measurement system scans across it and reads out the actual linewidth, typically in microns or some related scaled units. Thus by statistically sampling different locations on a wafer a decision as to whether to reject, reprocess or move on to further processing can more readily be made. Clearly, then, with the need for repeated and extremely precise inspection there has long been a need for a fully automated wafer inspection system that quickly extracts a chosen wafer from a supply, positions a given zone within a given one of many patterns on the wafer at the inspection region, enables an automatic measurement to be made or visual inspection to be undertaken without additional steps, and then transfers the inspected wafer to an appropriate storage category.
Despite this long felt need, there are many reasons that suitable systems meeting these requirements have not heretofore become available. Using a large precision mechanism can itself introduce substantial contamination, as well as increase costs disproportionately to the inspection rate of an individual inspector. The handling mechanism must thus be free of frictional abrasive effects, and retain the wafer in such a way that a coating, as a photoresist coating, on the top and edges of the wafer is totally undisturbed. It must not only position the wafer in fractions of a micron in X and Y directions, but must also rotate it in accordance with the orientation of a line or area to be inspected. A further problem arises from the instability of the viewed image. At the .magnification that must now be used for inspection of high resolution patterns, the prior art does not provide an adequately stable platform for the microscope, and every vibration is transmitted because of the cantilevered mount used for the microscope.